Ed,
I know what you mean - I raised an enhancement request for exactly this issue. Unfortunately you can only define preferred artifact types and cannot exclude artifact types from being added into a module. An end user can add ANY artifact type defined in a project into ANY module, with potentially disastrous consequences for your workflow, printing templates, etc.
If you want to vote for it, you can do it here --> https://www.ibm.com/developerworks/rfe/execute?use_case=viewRfe&CR_ID=100761

Donald,
The use case is, that you can associate artifact types such as 'Stakeholder Requirement', 'Heading' with a module artifact, say 'Use Cases'. When a user editing a module of artifact 'Use Cases' inserts a new artifact they can select any artifact type into the module even if it makes no sense at all for that module, eg insert an artifact of type 'Test Case' into the module 'Use Cases'. What we need is a deny list/allow list concept, to restrict which artifact types can be added into the special artifact type of module.

Knock-on effects of not being able to restrict the artifact types per module are:
- View creation/change .... any attribute of any artifact type is visible in any module type when  searching for attributes to add to a view. You can add any of these attributes into your view, when they are not associated with any artifact tyes in the module but nothing will ever be displayed in that view column.
- Exporting eg via RPE... artifact types not catered for in your RPE template but existing in a module you are exporting are ignored (or you have to have a condition which looks for any artifact type in in your expect list and print an error to say 'unexpected artifact type' or similar...
there are plenty of other circumstances where this lack of control creates problems.